DESCRIPTION:

Fill the connected area starting at x,y,z. If no sourceset is
given, or it is empty, use the class of the voxel at x/y/z.
All voxels belonging to sourceset that are connected to x,y,z
are remarked to targetset.
If maxsteps is given, the flooding is stopped after this number
of steps.
The difference to MARKCC_CONNECTED is, that progress is displayed
after every round, which allows for interactive control but may
be slow if used on slowly-updating views.